1. Field of the Invention
The present invention relates, generally, to a clutch assembly, and more specifically, to a start-up clutch assembly for translating torque between a prime mover and a transmission.
2. Description of the Related Art
Generally speaking, land vehicles require a powertrain consisting of three basic components. These components include a power plant (such as an internal combustion engine), a power transmission, and wheels. The power transmission component is typically referred to simply as the “transmission.” Engine torque and speed are converted in the transmission in accordance with the tractive power demand of the vehicle. Hydrokinetic devices, such as torque converters, are often employed between the internal combustion engine and its associated automatic transmission for transferring kinetic energy therebetween.
Torque converters typically include impeller assemblies that are operatively connected for rotation with the torque input from an internal combustion engine, a turbine assembly that is fluidly connected in driven relationship with the impeller assembly, and a stator or reactor assembly. These assemblies together form a substantially toroidal flow passage for kinetic fluid that circulates in the torque converter. Each assembly includes a plurality of blades or veins that act to convert mechanical energy to hydrokinetic energy and back to mechanical energy. The stator assembly of a conventional torque converter is locked against rotation in one direction but is free to spin about an axis in the direction of rotation of the impeller assembly and the turbine assembly. When the stator assembly is locked against rotation, the torque is multiplied by the torque converter. During torque multiplication, the output torque is greater than the input torque for the torque converter. However, when the stator assembly freewheels in the direction of rotation of the impeller and turbine assemblies, there is no torque multiplication and the torque converter becomes a fluid coupling. Fluid couplings have inherent slip. In the absence of a fully engaged lock-up clutch, torque converter slip exists when the speed ratio is less than 1.0 (RPM input>RPM output of the torque converter). This inherent slip reduces the efficiency of the torque converter.
While torque converters provide a smooth coupling between the engine and the transmission, the slippage of the torque converter results in parasitic losses that decrease the efficiency of the entire power train. More specifically, the operating efficiency of the converter during start-up is relatively low. It varies from a zero value at stall to a maximum value of approximately 80-85% at the coupling point. The coupling point occurs at the transition from the torque multiplication mode to the coupling mode when the torque multiplication ratio is unity.
In addition to the problems with efficiency, torque converters of the type known in the related art occupy substantial space in the driveline assembly between the transmission gearing and the engine. Torque converters typically define relatively large diameters when compared to the transmission gearing. Further, the torque converter has a substantial rotating mass that must be accelerated by the engine during start-up of the vehicle during forward drive or in reverse drive. The effective mass of the converter necessarily includes the mass of the hydraulic fluid that circulates in the torus circuit defined by the converter impeller, the turbine, and the stator assembly.
On the other hand, frictional clutches have been also employed in the related art to selectively connect a source of rotational power, such as the crank shaft of an internal combustion engine and its flywheel, to a driven mechanism, such as a transmission. The frictional clutches of the type that have been employed in the related art overcome the disadvantages associated with reduced efficiencies, parasitic losses, relatively large effective mass and the space that is occupied by torque converters used for the same purpose. In an automotive context, clutches used for this purpose are often referred to as “start-up” clutches. Clutches of this type typically include a clutch pack that is operatively supported between a drive and driven member of the clutch assembly. The drive member is operatively connected to the torque input from the prime mover. The driven member is operatively connected to the input shaft of the transmission. A piston assembly is supported for axial movement in response to hydraulic pressure to engage the clutch pack to translate torque between the drive and driven members. However, when the engine is connected with the transmission through friction clutches of the type known in the related art, vibrations are often transmitted through the clutch and into the transmission as well as other drivetrain components, producing undesirable noise conditions such as gear rattle. In addition, over time, torque spikes that can reach relatively high magnitudes at drivetrain resonance frequencies can damage the transmission as well as other drivetrain components.
As an alternative to the use of a hydraulically actuated piston, it is known in the related art to employ a spring biased piston assembly that biases the piston into engagement with the clutch pack such that the start-up clutch assembly normally operates in “closed pack” mode. A return mechanism is employed to disengage the piston assembly when torque translation through the drive and driven members is to be interrupted. While start-up clutches of this type have generally worked for their intended purposes, it has been found that pistons that are spring biased to engage the clutch pack typically require 8-12 mm. of travel in order to disengage the piston so that the clutch operates in “open pack” mode. Unfortunately, it can be difficult to design an appropriate spring-based biasing member that provides adequate load during “closed pack” operation of the clutch pack while, at the same time, allowing for open pack operation to interrupt torque translation in a relatively short amount of piston travel.
Accordingly, there is a need in the art for an improved start-up clutch assembly that effectively translates torque between the prime mover and the transmission in a cost-effective manner and that can be controlled so as to reduce or eliminate undesirable vibrations and torque spikes between the prime mover and the transmission. In addition, there remains a need in the art for an improved start-up clutch assembly that operates in a normally closed pack mode, provides adequate apply force through the piston, and that can be quickly shifted to “open pack” mode to interrupt torque translation between the drive and driven members with minimal movement of the apply piston.